Optical fiber strain sensors have been the topic of intense research during the last decade as they can be made very small, compact, immune to electromagnetic interference, biocompatible, and can be used at elevated temperature or in relatively harsh chemical environments. Applications for such sensors are, therefore, numerous and may range from structural monitoring to biomedical applications.
Certain fiber optic strain sensors are known in the art. One presented solution includes a specially designed double core fiber. Optical power is exchanged between the two cores as a function of applied strain. However, considerable lengths of optical fiber and complex signal processing may be required to make such systems practical. Other solutions are based on bend loss phenomena in the optical fiber where the applied strain modulates the intensity at a sensor output. For those solutions a relatively simple detection scheme may be applied, but they may have drawbacks in terms of relatively low sensitivity, relatively large size, and relatively low absolute accuracy that is typical for intensity-based sensors based upon bend loss phenomena.
Currently many fiber optic strain sensors are based on fiber Bragg gratings. Bragg grating sensors may however suffer from relatively high temperature sensitivity, require complex signal processing, and may not be miniature in their size (e.g., length). Other solutions to the optical fiber strain sensors rely on polarization effects in optical fibers and measurements of light pulse propagation time in a fiber that is exposed to a measured stress.
Certain strain sensing solutions can be obtained in the form of fiber-based Fabry-Perot sensors. Such fiber-based Fabry-Perot sensors may be appropriate for practical sensing applications since they may be interrogated by variety of straightforward and cost effective, commercially available opto-electronic interrogation techniques. In some applications, a Fabry-Perot interferometer may be used for strain measurements. For example, in the prior art, two perpendicularly cleaved optical fibers may be placed in a glass capillary in such a way to form the short air cavity between the fiber ends. This cavity creates an optical Fabry-Perot resonator that changes its length proportionally to expansion of the glass capillary. One drawback of this approach is in the use of adhesive and not well-defined point where fiber adheres to the capillary as the adhesive randomly penetrates the gap between fiber and capillary. Furthermore, the adhesion between capillary and fiber may impose thermal, mechanical, and chemical stability limitations. The outer diameter of such sensor is always considerably larger than the fiber diameter, which increases its size and may limit its possible applications and packaging options. Friction between the fiber and the capillary may cause sensor hysteresis. Furthermore, the production process of such capillary sensors may be complex and may involve number of precision alignment steps.
In another prior system, a hollow core optical fiber is used to create a spacer between two perpendicularly cleaved fibers. In this case, the sensor (hollow core optical fiber) has the same diameter as optical fibers but it relies on manufacturing and splicing of the hollow core optical fiber to a standard optical fiber which presents its own difficulties. In other embodiments, solutions including a concave cavity are used instead of hollow core fiber or capillary. In some embodiments, the creation of a strain sensitive cavity may be provided by an etching process. While such prior art optical sensors may eliminate some of the drawbacks of capillary-type sensors, they may suffer from limited sensitivity.
In prior art all-fiber designs, the cavity length is varied and converted into cavity length change under influence of an applied strain. The sensor sensitivity to the strain can be incensed by increasing the cavity length. However, this may lead to high optical losses, low interference fringe visibility and overall sensor signal degradation. In one solution, an air cavity is replaced by a fiber that can guide the light. For example, a section of single mode fiber is inserted between two semi-refractive mirrors to create a Fabry-Perot interferometer. While such solution allows for arbitrary resonator length, it may suffer from high temperature sensitivity induced by fiber core refractive index temperature dependence and may require a complex manufacturing procedure that involves vacuum deposition of mirrors onto the individual fiber surfaces. Furthermore, long Fabry-Perot cavities may require more complex signal processing techniques as the free spectral range of long cavity becomes narrow, requiring higher resolution spectrometric sensor signal interrogation techniques.
Furthermore, many, if not all, fiber Fabry-Perot strain sensors known in the art are made by complex and expensive production procedures that involve multiple production steps, and that are therefore not generally suitable for high volume, cost-effective production.
Thus, it should be recognized that the performance of such optical sensor devices may be limited, and/or manufacturing of such optical sensor devices may be relatively complicated and not cost effective. Therefore, there is a long felt and unmet need for highly effective optical sensor and manufacturing methods thereof.